MINOR: logs: Get the multiplexer exist status when no stream is provided

MINOR: logs: Get the multiplexer exist status when no stream is provided

When a log message is emitted from the session level, by a multiplexer,
there is no stream. Thus for HTTP session, there no status code and the
termination flags are not correctly set.

Thanks to previous patch, the HTTP status code is deduced from the mux exist
status, using the MUX_EXIT_STATE ctl param. This is only done for HTTP
frontends. If it is defined ( != 0), it is used to deduce the termination
flags.